How can I average two meshes together?
Do you have 1-1 vertex correspondence? If so, with both meshes selected, you could try adding one mesh as a shape key on the other.:



Then, move the slider to apply the shape key at 50%:



If your meshes don't have the same number of vertices, you can use a use a shrinkwrap modifier to warp one mesh onto the other before the shape key. This will make it match the other as closely as possible (consider this 100%). Then, you can apply that shrinkwrap as a shape key and turn the shape key back to 0.5 (consider this %50). Rinse and repeat for additional versions of the mesh.